Dual deploy sled set up questions.

[solid_fuel]

This will be my first attempt at a dual deploy. The rocket is the 3” Twitch Dual Deploy from Madcow. This is how I’m planning on laying out the Altimeters, switches and batteries on the sled. Does this look good? Any additional feedback? I am not fully committed to the pull pin switches quite yet; I’m not entirely certain on their use. I assume I have to put the eBay together then I can insert the switch to disarm them. And at this point I start filling the charge wells? Any and all feedback welcomed. Thanks.

 

[tim cubbedge]

I think your comment about pulling pins & arming altimeters in order to assemble av bay is a valid point and has been discussed here before. Lately I have been using twist & tape and when room allows, a wifi switch. Also, I prefer the duracell 9V alkaline as opposed to other 9V batteries. I believe I have read here & elsewhere that the cells are actually soldered together & not so on other 9V batteries.

 

[solid_fuel]

The batteries shown in my picture are for size purposes only. I will be using brand new Duracell 9V batteries. I also will be drilling the sled and using wire ties in a cross formation to secure the batteries. I am also thinking I might move the switch to the middle of the sled between the two batteries.

 

[mpitfield]

Logistically you are correct in the order of steps. Are you planning on connecting the e-matches directly to the altimeters or hard-wiring the altimeters to the bulkheads then using something like screw terminals to connect the e-matches to? I have done it both ways and in your case they have pluses and minuses. One thing that can help you is to key one bulkhead to teh AV Bay coupler so it only assembles one way and everything lines up with little effort.

The plus for screw terminals is that you can seal up your AV bay, disarm the electronics before you connect anything pyro to any the channels. The negative is that adding a screw terminal to your bulkhead that your altimeters are hard wired to introduces a point of failure. One bulkhead would have to be removable so the wires that connect that bulkhead to the altimeter would for convenience use quick disconnects.

Connecting directly to the altimeters means you have to pass the e-match wires through the bulkhead, one easy way to do this is to use one of the rubber grommets that has a screw in it and as you tighten it expands to seal the bulkhead. I have a bunch and can't recall where I got them but they are very easy to use, very reliable and inexpensive.

BTW I use both those altimeters with LiPos, which I prefer over alkaline battery chemistry. Obviously both will work but LiPos have the ability to deliver a lot of power at once and back to back due to their much lower internal resistance. This can be a blessing and a curse as they could easily damage your altimeters if there is a short. However LiPo does represent a bigger buy-in and learning curve to get you up and going.

One last thing, when arming with that single pin double switch SPST setup, it might help when arming if you pull the pin out to arm one altimeter at a time so you can clearly hear the continuity beeps on the pad. You could easily mark the pull pin so when the mark shows you stop to allow the altimeter to come up fully then pull the pin right out to arm the second. At least you will be able to clearly hear the first one. The good news is those two altimeters sound very different.

 

[solid_fuel]

My other battery choice is the Turnigy Nanotech 2S Lipo. I would need to buy a few more of the batteries since the ones I have I plan on using with the tracker but if IRC they arent that much compared to Duracell 9V batteries since I already have the charger. I haven’t settled on a way to get the leads for the e match through the bulk head yet. If I stick with the pull pin switches I will probably look at some kind of through bulkhead terminal. I have the Dog House charge wells ordered in case I do decide to go that route. Also I like the idea of the marking the pin to only arm one Altimeter at a time.

 

[solid_fuel]

If I go with the LiPo battery would it be better to have them on the sled like this?
Or like this? Or does it even matter?

 

[mpitfield]

If I go with the LiPo battery would it be better to have them on the sled like this? View attachment 369707
Or like this? Or does it even matter? View attachment 369706

Normally, but not necessarily you would use the pull pin hole as one of your sampling holes for the altimeters. If this is the case then again normally but not always the holes would be drilled in the AV bay band, if you are using one. Once again to flame proof the response, normally but not always that AV bay band is centered on the AV bay coupler. If this is the case then the switch position may have less flexibility and might need to be centered on the sled, so it would end up splitting the batteries.

Bottom line where you lay things out has to do with more than what we see posted so far. Can you provide more details on how the AV bay is configured on the airframe?

 

[solid_fuel]

The av bay is slightly more than half way up the rocket; little more than 24” from base. av bay is much longer than the sled it self. I will be using a band. I do plan on drilling sampling holes in the band one of which I will be using as an access port for the pull pin. When I do the switch band does it have to be exactly centered on the av bay or can it be slightly off center as shown in this picture?
Also what do I do to keep the sled from sliding back and forth on the threaded rods? I assume I use some nuts tightened up to the holes the rods go through is that correct? It seems like I will have much empty length in the avbay as the sled is barely half the length of the bay.

 

[mpitfield]

The av bay is slightly more than half way up the rocket; little more than 24” from base. av bay is much longer than the sled it self. I will be using a band. I do plan on drilling sampling holes in the band one of which I will be using as an access port for the pull pin. When I do the switch band does it have to be exactly centered on the av bay or can it be slightly off center as shown in this picture? View attachment 369716

The general rule of thumb for the amount of coupler in the airframe is 1 caliber, so if it's a 3" airframe then 3" in the tube. This is just a general rule and is designed to ensure a min amount of structural strength in that location. IMO what you show in your pic would be completely fine.

Also what do I do to keep the sled from sliding back and forth on the threaded rods? I assume I use some nuts tightened up to the holes the rods go through is that correct? It seems like I will have much empty length in the avbay as the sled is barely half the length of the bay.

I most commonly use two nuts on either end of the threaded rods to lock the sled in place, one nut just locks the other. I have also used a single nut with red loctite. Another method I have used is to lock the sled to one bulkhead using supports and epoxy, a little bit more permanent. Like a lot of things in this hobby there is more than one way to accomplish this.

With a relatively short sled in a long AV bay it does allow you a bit of flexibility as to where you lay things out so it will likely come down to preference more than anything.

 

[David Schwantz]

Doing my first DD also. I was told to put your batteries at the bottom of the sled when standing on pad. I made my sled just the right length so that it would not slide around. I wanted my Black Hole to be a "convertable" so I have made it to fly as motor eject with no payload section and also DD with payload section added. I put alignment pins made out of 4-40 socket head bolts, drilled holes in matching parts of AV bay and the NC shoulder. All parts fit together with only using the same hole in booster.

 

[cwbullet]

That pin is unique. I have seen folks use a hole but not a screw head.

 

[Banzai88]

That pin is unique. I have seen folks use a hole but not a screw head.

Fiberglass rods or wooden dowels are easier to work with, and there's a few of us that have been doing it for a while.

 

[Handeman]

I generally only use one altimeter for 4" and smaller rockets. I put the battery on one side, terminal down, on the upper part of the sled. The altimeter goes on the bottom of the sled opposite of the battery. (I'm still using an altimeter that survived a user-error induced lawndart, the battery was poured out of the remains of the av-bay but the altimeter survived).

As for pull pins, I use them in the vent band. I also put screw switched that are accessible on the av-bay shoulder where the separation occurs. That way you can leave everything turned off until you are ready for final assembly. Put the pins in and turn on the screw switches, put the rocket together, insert shear pins if you use them, and take it to the RSO. No power is applied until it is on the pad and ready to fly. (I do recommend turning power on and testing continuity of the matches after they are connected, but before the powder is added, by letting the altimeter go through it's power up sequence so you get the "beeps" of continuity.)

If you are going to use two altimeter, I would recommend the batteries be on opposite sides of the upper part of the sled, terminals down. The two altimeters on the bottom. That makes it easy and clear how to wire everything in totally separate circuits.

 

[GrouchoDuke]

Fiberglass rods or wooden dowels are easier to work with, and there's a few of us that have been doing it for a while.

Wow, that's pretty. I bet that works great for aligning the shear pin holes.

 

[Buckeye]

I assume you are using threaded rods? You will need to clear those when attaching your components to the sled.

The thing I don't like about the pull-pin switch is that the electronics are turned "on" while you are trying to assemble the bay. Plus, they are pretty large in size.

Personally, I would put that second altimeter in another rocket and double my fun. For your first DD, keep it simple. The human is more likely to fail, not the altimeter.

 

[David Schwantz]

Fiberglass rods or wooden dowels are easier to work with, and there's a few of us that have been doing it for a while.

I don't know about easier, with mine all I did was drill hole and bolt it in. But yours do look very nice. Think I might try that on this next one. How far do they extend on the inside to give them enough support?

 

[Banzai88]

Wow, that's pretty. I bet that works great for aligning the shear pin holes.

Yes. I do it on all of my fiberglass and phenolic rockets. Recently started doing it with nose cones, too. It makes sure that everything lines up perfectly, every time.

 

[Banzai88]

I don't know about easier, with mine all I did was drill hole and bolt it in. But yours do look very nice. Think I might try that on this next one. How far do they extend on the inside to give them enough support?

Only needs to be just long enough to hold some glue. I typically cut the pieces about 1/8-3/16" long, slather with epoxy on the inside and leave enough sticking out on the outisde to sand smooth with just a few passes of the sandpaper during paint prep. Once the epoxy has cured you can also trim it flush inside if you want, but that little 1/16" isn't ever really enough to interfere with anything.

 

[David Schwantz]

I have a 4-40 lock nut on the inside of mine with no threads sticking out into the bay. Did not want anything to get caught up on it. I also like how you did the 2 & 1 pin method. But I did not put a switch band on mine.

 

[Handeman]

Only needs to be just long enough to hold some glue. I typically cut the pieces about 1/8-3/16" long, slather with epoxy on the inside and leave enough sticking out on the outisde to sand smooth with just a few passes of the sandpaper during paint prep. Once the epoxy has cured you can also trim it flush inside if you want, but that little 1/16" isn't ever really enough to interfere with anything.

Tom,

You might have been the person I picked that technique up from. It it was you, thanks.....

I've been using it on my fiberglass rockets and it works great for shear pin alignment. I just push the pins in, never have to wiggle or pry to rocket to get the holes to align. I use it for the av-bay to tube alignment, nose cone to tube, and even to align the caps of the av-bay to the tube so the sled always lines up and the pull pin holes are always right.

 

[Cameron Anderson]

I have used a mix of switches - push button, 2 position, screw...and I still use them all based on the airframe and the altimeter. I mount my 9Vs mounted vertically with the connections down to eliminate launch forces from disconnecting them, which also has inherent centrifugal resistance if I have a wanted fin (a friend in November lost a nice airframe when excessive spin disconnected him horizontally mounted 9V and the whole thing became a poorly designed BLU-109.